A key method is known as carbon/nitrogen (C and N) stable isotope analysis.

Source: https://www.salon.com/2023/10/25/people-in-europe-ate-seaweed-for-thousands-of-years-before-it-largely-disappeared-from-their-diets_partner/

And I'm not an expert in chemistry, but I guess from the isotope ratio or the mixture of elements, they concluded it was not made on Earth.'

Source: https://www.dailymail.co.uk/news/article-12838131/Secrets-Area-51-Metallic-egg-shaped-UFO-1980s-whistleblower-claims.html?ns_mchannel=rss&ns_campaign=1490&ito=1490

Earlier, local broadcaster Channel 10 had reported mercury and a uranium isotope were discovered.

Source: https://www.bbc.co.uk/news/world-australia-66530479?at_medium=RSS&at_campaign=KARANGA

In 2019, we were able to position-specific isotope analysis for several different mammals.

Source: https://phys.org/news/2023-09-body-food-fuel-tracked-atoms.html

Record concentrations of a helium isotope found inside 62-million-year-old Arctic rocks could be the most compelling evidence to date of a slow leak in our planet’s core.

Source: https://www.imao.us/2023/10/straight-line-of-the-day-oh-no-our-planet-is-leaking-why-and-what-do-we-do/

The environment ministry’s tests of samples taken from 11 points near the plant concluded concentrations of radioactive isotope tritium below the lower limit of detection – 7 to 8 becquerels of tritium per liter.

Source: https://newsinfo.inquirer.net/1822575/japan-says-seawater-radioactivity-below-limits-near-fukushima

The proportion of the radioactive carbon-14 isotope that is absorbed by living organisms into their tissue decays over time, which allows scientists to determine the approximate year of death of the specimen.

Source: https://gazette.com/news/us-world/exclusive-a-close-encounter-with-the-alien-bodies-in-mexico/article_25ffd68d-0160-59d6-88b5-4ad728e29339.html

This isotope could provide nuclear energy in a fusion reactor but since it is not radioactive it would not produce dangerous waste, according to the European Space Agency.

Source: https://www.channelnewsasia.com/business/explainer-moon-mining-why-major-powers-are-eyeing-lunar-gold-rush-3693376?cid=cna_flip_070214

This isotope ratio allows reviewing the relation of the trees' rate of photosynthesis and the stomatal conductivity, i.e., the photosynthetic rate of trees in comparison to water use.

Source: https://phys.org/news/2023-03-fast-recovery-suppressed-norway-spruce.html

Tritium is a hydrogen isotope in the water molecule, but it decays to a helium isotope, not to hydrogen.

Source: https://japantoday.com/category/politics/japanese-ministers-eat-fukushima-fish-to-show-it%27s-safe-after-nuclear-plant-wastewater-is-discharged

Tritium is a hydrogen isotope in the water molecule, but it decays to a helium isotope, not to hydrogen.

Source: https://japantoday.com/category/politics/japanese-ministers-eat-fukushima-fish-to-show-it%27s-safe-after-nuclear-plant-wastewater-is-discharged